Dereferencing

Accessing a member of a variable through the dot operator.

• Dereferencing a null variable whose value is null is a NullReferenceException.

Checking for Null

void FindRoot(Node node, Action<Node> processNode) {
	// If this wasn't checked first…
	for (
		var current = node; 
		current is not null; 
		current = current.Parent) { // …then this could be a NullReference…
		processNode(current); 	  // …and so could this.
	}
}

Also:

if (message is not null) { … } 

Using Utility Methods to Check for Null

If you have a private utility method like IsNotNull() to check for null, you must add an attribute to its signature to tell the compiler what it does:

private static bool IsNotNull([NotNullWhen(true)] object? obj) => obj != null;

Null-forgiving Operator (!)

The null-forgiving operator (!) forces the null-state of a variable to be not-null.
Use this when you know the variable cannot be null:

name!.Length;

or

string msg = TryGetMessage(arg)!;

Null-conditional Operator (?.)

The null-conditional operator (?.) assigns null to a variable instead of throwing an exception:

string authorName = null;

This throws a NullReferenceException:

int x = authorName.Length

If authorName.Length is null, set y to null. Else, set y to authorName.Length:

int? y = authorName?.Length;

Null-coalescing Operator (??)

The null-coalescing operator (??). If the left-side operand is not null, return its value. Else, evaluate the right-side operand:

If authorName.Length is null, set y to 3. Else, set y to authorName.Length:

int y = authorName?.Length ?? 3;

The right side can also be a throw statement:

var name = value ?? throw new ArgumentNullException(nameof(value), "Name cannot be null.");

Null-coalescing Assignment Operator (??=)

The null-coalescing assignment operator (??=). If the left-side operand is null, assign it the value of the right-side operand.

int a? = null;
List<int> numbers = new();

numbers.Add(5);
// If a is not null, Add(a).  Else, Add(0).
numbers.Add(a ??= 0); // Adds 0 to numbers.

This operator can replace null checking if statements. So, instead of:

if (variable is null) { 
	variable = expression;
}

Use:

variable ??= expression;

Avoid Non-nullable Reference Variables Being Uninitialized

public class Person {
	public string FirstName { get; set; }
	public string LastName { get; set; }
	
	public Person(string f, string l) {	// This constructor requires FirstName and LastName, thereby avoiding a
		FirstName = f;			  // nullable reference.
		LastName = l;
	}
}

If the object is required to be created before it’s properties are set, use a default non-null value:

public class Person {
	public string FirstName { get; set; } = string.Empty
	…
}

Allow Reference Types to Hold Null Value

By default in .NET 6+, reference types cannot hold null values. To override this, at the top of a code file:

#nullable disable